Field of the Invention
Embodiments of the invention relate generally to the field of memory devices and more particularly, to reducing the effect of random telegraph signal noise (RTS noise) in semiconductor memories.
Description of the Related Art
Flash memory is a non-volatile memory that can be electrically erased and reprogrammed. It is primarily used in memory cards, USB flash drives, and the like for storage of data in computer systems. Generally, flash memory stores information in an array of floating gate transistors, called “cells”, each of which traditionally stores one bit of information that is represented as a “0” or a “1”. Each cell is characterized by a threshold voltage (Vt) that varies based on the data stored in the cell. For example, during program and erase operations, charge is added or removed from a floating gate to change the cell's threshold voltage, thereby defining whether the cell is programmed or erased. During a read operation, a read voltage is applied to the cell and a response of the cell (e.g., a current across the cell) is monitored to determine whether the threshold voltage is above or below the read voltage. In other words, the read operation can determine if the cell is programmed as a 1 or a 0 value. Multi-level cells may include multiple threshold voltage ranges that are representative of additional values, such as two or more bits of information.
Flash memories may also employ a verify operation that ensures that each cell is programmed as a given state, such as a 1 or a 0. The verify operation may provide a sufficient margin between the 1 and 0 states such that a cell is charged to a given range and does not charge to an intermediate state where the cell may be read incorrectly. However, when the memory cells are programmed at one temperature and are read out at another temperature, the margin may decrease, potentially causing the value of the cell to be read incorrectly. For example, when the read operation is executed during a first period at a first temperature, the data may tend to be a 0, and when the read operation is executed during a second period at a second temperature, the data may tend to be a 1. This is prevalent where a word line voltage is a constant voltage value over the range of temperatures. The variations may be attributed to random telegraph noise (RTS noise) and the resulting time dependency of the current that passes through the memory cell. The RTS noise can be attributed to the trap and detrap of electrons or the recombination of electrons with holes. Unfortunately, the presence of the RTS noise and the resulting inaccuracies in reading the memory cells may produce inaccurate and/or less reliable memory devices.
Embodiments of the present invention may be directed to one or more of the problems set forth above.